1. Field of the Invention
The present invention relates to a condenser lens system used in an illumination optical system for a microscope. More particularly, the invention relates to a condenser lens system used in illumination optical system for an inverted microscope.
2. Related Background Art
FIG. 11 is a drawing to show a schematic structure of an inverted microscope. The structure of the inverted microscope is first described briefly. Light from a light source 10 is collected and condensed through a collector lens 11 and a condenser lens 12 to illuminate a sample O cultured in a culture solution W in a laboratory dish S. The dish S is mounted on a stage 13 which is two-dimensionally movable. An objective lens 14 and a reflecting mirror M.sub.1 focus light passing through the sample O in the dish S to form a spatial image I.sub.1. Then the spatial image I.sub.1 is relayed by a relay lens 15 and a reflecting mirror M.sub.2 to form another spatial image I.sub.2. After that, the light is guided through a path-bending prism P to an eyepiece 16, whereby the spatial image I.sub.2 (image of sample O) is observed through the eyepiece 16, as enlarged.
The inverted microscope as shown in FIG. 11 is frequently used for observing a sample cultured in a culture solution W in a laboratory dish S. For such an observation, an illumination system for an inverted microscope employs as the condenser lens 12, for example, a condenser lens system LS as shown in FIG. 12, which has a long operational distance (distance between an illuminated surface and the condenser lens), which is disposed away from a liquid surface of the culture solution W, and through which the light from the illumination light source 10 illuminates the sample O in solution.
In case the sample O in the culture solution W is illuminated using the condenser lens system LS having a long operational distance as shown in FIG. 12, external vibration causes the liquid surface of the culture solution W to fret, which in turn causes illumination unevenness, resulting in greatly interfering with the observation of the sample O. Such a problem occurred frequently with the condenser lens system LS as shown in FIG. 12.
In particular, if the sample O in the culture solution W is observed using an inverted phase-contrast microscope or an inverted differential interference microscope, which detects a slight phase difference between rays passing through the sample O, a slight shake of the liquid surface of the culture solution W produces a phase difference at the liquid surface. This phase difference at the liquid surface becomes a great disturbance in the observation of the sample O in the culture solution W. This is a problem.
When manipulation is made to manipulate a sample in the culture solution W with a slender glass tube, the surface tension of the culture solution W raises the liquid surface in the contact portion between the slender glass tube and the liquid surface. This portion causes a phase difference, which makes the observation of the sample O through the inverted phase-contrast microscope or the inverted differential interference microscope very difficult during manipulation. This is also a problem.
Also, since the condenser lens system LS having a long operational distance as shown in FIG. 12 inevitably has a long focal length in order to assure the long operational distance, it is difficult to give a large numerical aperture to the condenser lens with respect to the optical design. In case the sample in the culture solution is observed using an objective lens with a high magnification and a large numerical aperture, the intrinsic performance of the objective lens with the large numerical aperture cannot be fully utilized.